Frequency-modulating apparatus



' E. J. GORN FREQUENCY-MODULATING APPARATUS Filed April 22,. 1947 April25, 1950 /NVENTO/? Patented Apr. 25, 1950 FREQUENCY-MODULATING APPARATUSElmer J. Gorn, Newton, Mass, asslgnor to Raytheon Manufacturing Company,Newton, Mass..

a corporation of Delaware Application April 22, 1947, Serial No. 743,043

' 2 Claims.

This invention relates to frequency-modulating apparatus, and moreparticularly to frequency-modulating apparatus for electromagneticenergy in the microwave region of the frequency spectrum.

One of the objects of the present invention is to accomplish frequencymodulation of microwave energy by altering the velocity of propagationthereof under the control of a modulatingsignal containing theintelligence it is desired to transmit.

Another object of the present invention is the provision of simple andefiicient apparatus to effectuate the foregoing purpose.

- density. Said space charge may take the form of an electron cloudadapted to move between an electron-emissive electrode, located withinthe waveguide and functioning as a cathode, and the longer side walls ofthe waveguide itself, functioning as an anode. In order to obtain aspace charge of appreciable density, it isdesirable to prevent theelectrons from being swept out of the effective region of the waveguidetoo rapidly, and for this purpose, it is preferred that a magnetic fieldbe established through the waveguide, in a direction at an angle to thecharge path between the emissive electrode and the waveguide itself.Said magnetic field, in conjunction with an electric field set upbetween the emissive electrode and the waveguide, causes the emittedelectrons to travel along spiral rather than direct paths, therebyretaining said electrons in the effective region of the waveguide forlonger periods of time.

Now, the density of the initial space charge will determine thedielectric constant of the waveguide and, as is well known, saiddielectric constant will determine the velocity of propagation of theelectromagnetic energy through said waveguide. Hence, by' varying thedensity of said space charge, for example, under the control of anintelligence-containing modulating signal, the velocity of propagationand, therefore, the instantaneous phase, of the initial energy will bealtered, such variation in instantaneous phase constituting,effectively, a variation in frequency. An increase in the space chargedensity will increase the dielectric constant of the waveguide,

reduce the velocity of propagation of the energy therethrough, and lagthe phase of said energy to result in an instantaneously lowerfrequency: while a reduction in the space charge density will reduce thedielectric constant'of the wave- 'gulda increase the velocity of thepropagation of the energy therethrough, and advance the phase of saidenergy to result in an instantaneously higher frequency.

In the accompanying specification there shall be described, and in theannexed drawing shown,

an illustrative embodiment of a frequency modulating apparatus made inaccordance with the present invention. It is, however, to be clearlyunderstood that the present. invention is not to be limited to thedetails herein shown and described for purposes of illustration only,inasmuch as changes therein may be made without the exercise ofinvention, and within the true spirit andscope of the claims heretoappended.

In said drawing, the single figure is a perspective view of a preferredform of the invention.

Referring now more in detail to the aforesaid illustrative embodiment ofthe present invention, with particular reference to the drawingillustrating the same, the numeral 5 generally designates a waveguide,herein shown as a hollow, rectangular pipe made of electricallyconductive material, such as copper. Said waveguide includes longer sidewalls 6, shorterside walls I and an end wall 8, and may be coupled, atits open end, to a load, such as an electromagnetic horn (not shown).

The waveguide 5 may be excited with electromagnetic energy of a wavelength, preferably, of a few centimeters or less, by means of a probe 9entering said waveguide through one of the longer side walls 6 thereofat an appropriate distance from the end wall 8, said probe beingsuitably coupled to a source of said energy, herein shown as a'magnetronoscillator Ill.

At a distance of a few wave lengths from the probe 9, the waveguide 5is, preferably, hermetically sealed, for example, by a low-lossdielectric window II, the output end of said waveguide being coupled, inan appropriate manner, to the load which may, as before stated,consistof a radiating electromagnetic horn.

The purpose of sealing off a portion of the waveguide 5 is to'provide aregion which may be evacuated and within which the dielectric con-rstant of the waveguide may be controlled. For this purpose, a spacecharge may be introduced into said evacuated region, said space chargebeing obtained, for example, in the following manner. A pair of opposed,elongated electron-emissive electrodes l2 may be mounted within thesealed-off portion of the waveguide 5, parallel with and slightly spacedfrom the shorter side walls 1 thereof, said electrodes being heated to atemperature of thermionic emission in any suitable manner (herein notspecifically shown). Said electrodes 12 may be connected, as shown, tothe negative terminal of a unidirectional voltage source IS, thepositive terminal of said voltage source being connected to the body ofthe waveguide itself. The electric field thereby established between thecathode electrodes l2 and the body of the waveguide 5, functioning as ananode, causes the flow of an electron stream between said electrodes andsaid waveguide, thereby providing the evacuated region of the latterwith a space charge.

In order to obtain a space charge of appreciable density and therebyenable better control over the dielectric constant of the waveguide, itis preferred that a magnetic field be established through said waveguidein a direction at an angle to the space charge path between the cathodeelectrodes l2 and said waveguide. By so doing, the electrons in saidspace charge may, by appropriate initial adjustment ofthe relativemagnitudes of said magnetic field and the electric field set up by thevoltage source l3, be caused to follow spiral rather than direct paths,thereby maintaining said electrons in the eiiective region of thewaveguide for relatively extended periods of time. Said magnetic fieldmay be obtained by opposed pole pieces l4 mounted externally of thewaveguide adjacent to and parallel with the shorter side walls 1 of saidwaveguide.

In order, after a space charge of suitable density has been initiallyobtained, to vary the same under the control, as heretofore explained,of a modulating signal, there may be superimposed upon the electricfield set up by the voltage source l3 an alternating voltagecharacteristic of the intelligence it is desired to transmit. For thispurpose, a secondary winding i5 of an audio-frequency transformer l6 maybe connected in se- -ries with the waveguide 5, voltage source l3 andcathode electrodes l2, said transformer including a primary winding l1adapted to receive the modulating signal.

Now, upon the application of an alternating voltage to the transformerIS, the magnitude and frequency of the electric field between thecathode electrodes l2 and the waveguide 5 will be varied as a function,respectively, of the magnitude and frequency of said alternatingvoltage. This will vary the instantaneous space charge density, in otherwords, the dielectric constant, within the waveguide in step with thefrequency of the modulating signal; and this, in turn, will alter thevelocity of propagation through the waveguide of the energy fed theretoby the oscillator I0. Such alteration of the velocity of propagationcauses alteration of the instantaneous phase of the energy affected,which change in phase constitutes, effectively, a change in frequency.During positive excursions of the modulating signal, the space chargedensity and, therefore, the dielectric constant, will be increased toretard the phase of the microwave energy, corresponding to a lowering ofthe frequency thereof, and during the negative excursions of themodulating signal, the opposite phenomena will occur. Thus, themicrowave energywill become frequency modulated about its centerfrequency in accordance with the intelligence incorporated inthemodulating signal applied thereto.

This completes the description of the aforesaid illustrative embodimentof the present invention. I

It will be noted from all of the foregoing that there has been-provideda simple and eifective mechanism for frequency modulating microwaveenergy through alteration of the velocity of propagation thereof underthe control of a modulation signal containing the intelligence it isdesired to transmit.

Other objects and advantages of the present invention will readily occurto those skilled in the art to which the same relates.

What is claimed is:

l. Modulating apparatus comprising: a hollow rectangular wave guidehaving a portion thereof sealed off and evacuated; a fixed-frequencysource of microwave energy communicating with said sealed off portion ofsaid wave guide; a cathode disposed in said sealed off portion of saidwave guide, adjacent to and insulated from one of the shorter side wallsthereof; a source of unidirectional voltage connected between saidcathode and said wave guide for introducing into said sealed oif ortionof said wave guide a space charge of predetermined density; means,adjacent the shorter side walls of said wave guide,

' microwave energy through said wave guide; and

means for superimposing an alternating voltage on said unidirectionalvoltage in response to a modulating signal.

2. Modulating apparatus comprising: a hollow rectangular wave guidehaving a portion thereof sealed off and evacuated; a fixed-frequencysource of microwave energy communicating with said sealed 01f portion ofsaid wave guide; a. pair of opposed cathodes disposed in said sealed oi!portion of said wave guide, respectively, adjacent to and insulated fromthe shorter side walls of said wave guide; a source of unidirectionalvoltage connected between said cathodes and said wave guide forintroducing into said sealed oil. portion of said wave guide a spacecharge of predetermined density; means, adjacent the shorter side wallsof said wave guide, for establishing a magnetic field through saidsealed off portion of said wave guide, in a direction at an angle to thespace charge path between said cathodes and said wave guide and at anangle to the direction of propagation of said microwave energy throughsaid wave guide; and means, connected in series with said cathode andsaid source of unidirectional voltage, for superimposing on the latteran alternating voltage in response to a modulating signal.

EIMER J. GORN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,241,976 Blewett et al May 13,1941 2,256,608 Braden Sept. 23, 1941

